The present invention relates to a method for assembling semiconductor devices on a substrate, such as a PCB substrate (printed circuit board), using a flip-chip assembly technique.
Printed-circuit-board (PCB) assemblies, such as for telecommunications applications, are designed and produced with a high reliability as an important criterion. Therefore the technologies, applied in these assemblies, are often limited to well established and qualified technologies like reflow and wave soldering of standard SMT (surface-mount-technologies) and through-hole components. However, increasing demands for miniaturization, higher functionality, higher number of pins and higher frequencies put pressure on the telecommunications equipment manufacturers to introduce more advanced packaging technologies such as BGA (ball-grid-array), CSP (chip-size-package) and flip-chip assembly. These assembly techniques must cope with the requirements of, for example, larger chip size, increased number of pins or bonding pads, smaller pitch size, i.e. the distance between adjacent or neighboring pins or bond pads.
Flip chip bonding technologies provide a high package density and electrical performance compared to other LSI (large-scale-integration) packaging technologies. Several flip chip bonding techniques have been developed, mostly based on solder bonding of the chip to the substrate. According to the conventional flip chip assembly processes, the chip is attached to the substrate connecting the bonding pads by reflowed solder bumps. Afterwards the underfill material is dispensed alongside the edges of the bonded chip and allowed to flow in between the bonded chip and the interconnect substrate by means of capillary action. Finally, the underfill material is cured. The underfill material is the material used to fill the fixed stand-off gap between the chip and the substrate and in between the solder bumps connecting both parts.
In xe2x80x9clow-cost High throughput Flip Chip Processingxe2x80x9d by D. Baldwin et al. in xe2x80x9cAdvance Packaging Magazinexe2x80x9d, January 2000, an alternative flip chip solder based bonding technique is proposed. First, a controlled volume of underfill material is stencil printed over the bonding pads. Next, solder paste is printed onto the board or substrate. Then, chips or SMT components are placed onto the substrate, having the chip bumps aligned to the bonding pads, compressing the liquid underfill material until the components make sufficient electrical and mechanical contact with the substrate. Finally the solder bumps are reflowed and the underfill material is cured. Although this alternative partly eliminates the time-consuming step of having the underfill material flow from the edge underneath the bonded chip, the proposed process still uses lead-bearing solder to form an electrical connection between the SMT component and the PCB-substrate.
In modern electronic assemblies, attempts are made to replace Pb/Sn solder with a number of alternative materials. This trend is driven by environmental reasons but also by the need to assemble components having finer or smaller pitch size. Among these alternative materials, leadless solders and adhesives are the most prominent candidates. With respect to the adhesives one distinguishes anisotropic conductive adhesives (ACAs), isotropic conductive adhesives (ICAs) and non-conductive adhesives (NCAs). An ACA consists merely of an electrically isolating NCA, wherein conductive particles, e.g. Ni/Au coated resin spheres, are dispersed. The content of dispersed conductive particles is limited to a few weight percent, and the particles are not linked to form chains. These particles form, after curing, a conductive path in a direction perpendicular to the substrate and the bonded component. An ICA also consists of an NCA but contains a very high content of conductive particles, e.g. silver flakes. After curing of the adhesive, the ICA therefore conducts isotropically.
In xe2x80x9cEvaluation of Isotropic Conductive Adhesives (ICA) for Solder Replacementxe2x80x9d by R. Pernice et al. in xe2x80x9cInternational Society for Hybrid Microelectronics (ISHM)xe2x80x9d Proceedings 1994, pp. 561-565, adhesives are used to bond coarse pitch components such as chip capacitors and resistors on a standard FR4-laminate substrate. Several ICA""s were used to create an adhesive junction between the bonding lands on the substrate and the bonding pads of the SMT components under test. The adhesive junction is established by dispensing the adhesive on the bond pad metallizations, mounting the chip onto the bond pads and applying an appropriated heat and/or pressure process. The author reported non-consistent electrical stability or even mechanical failure of the adhesive junctions.
An aim of the present invention is to offer, in comparison to the state of the art, a simplified, reliable and cost-effective assembly process.
In particular, the invention aims to present a reliable assembly process, employing adhesives, applicable for all type of substrates, even for cheap, low-temperature substrates.
The invention further aims to offer a reliable assembly process, employing adhesives, applicable for mounting fine pitch components.
Finally, the invention aims to form units where both advanced (i.e. according to the invention) and classical (i.e. solder based) assemblies are present on the same PCB or other substrate. For such xe2x80x9cmixed assemblyxe2x80x9d units, the invention proposes an assembly technique compatible with the classical one. One can hence choose the most appropriate assembly technique for each specific component to be attached on a substrate.
The present invention relates to a process for manufacturing an apparatus comprising at least one substrate and at least one component. In one aspect of the invention, an underfill material is applied to the substrate prior to the fusing or bonding of the substrate to the component. Thereafter, the substrate is bonded or fused to the at least one component. Through the process of bonding, electrical contact is made between the substrate and the at least one component. In one embodiment, electrical contact is made between bonding pads on the substrate with bonding pads on the component. There are several methods of bonding the substrate to the component including exerting a mechanical pressure so that contacts on the substrate and contacts on the component are bonded using an isotropically conductive adhesive, i.e. ICA. An alternate method is using soldering to bond the substrate with the component. When soldering, mechanical pressure may also be used for bonding.
In another aspect of the invention, a process for manufacturing an apparatus comprising at least one substrate and at least one component is disclosed. The component is attached to and in electrical contact with said substrate, characterized in that said process comprises the following steps:
providing a substrate, comprising on its surface a plurality of metal areas, called bonding pads or contact pads,
providing a component, comprising a plurality of metal areas, such as pins or bumps, also called bonding pads or contact pads, said bonding pads on said component corresponding to at least one group of said bonding pads on said substrate,
dispensing an adhesive onto said bonding pads of said substrate or onto said bonding pads of said component,
drying said ICA,
applying an underfill material in a predefined pattern on an area situated between said bonding pads of said substrate,
aligning said component so that said bonding pads of said component are directly above one group of bonding pads of said substrate,
exerting a mechanical pressure on said component, while maintaining said mechanical pressure and said predefined distance, performing a curing step, also called a thermocompression step, to cure said adhesive (e.g., isotropically conductive adhesive) and said underfill material (NCA) thereby creating electrical contacts between said component and said substrate.
According to an embodiment, the adhesive used is an isotropically conductive adhesive, i.e. ICA.
Moreover, according to an embodiment, the step of exerting a mechanical pressure includes exerting a mechanical pressure on said component until a predefined distance is reached between said component and said substrate, whereby corresponding bonding pads (2a, 2b) of said component and said substrate are both in contact with said ICA.
According to an embodiment of the present invention, the material of said substrate is chosen from the group consisting of FR4, FR5, glass-epoxy materials, ceramic materials, polyimide, polyester, alumina based materials and plastic materials.
According to an embodiment of the present invention, said component is chosen from the group consisting of a packaged semiconductor chip, a non-packaged semiconductor chip, a naked semiconductor die and a component made of a non-semiconductor material.
According to an embodiment of the present invention, said ICA is a non-conductive epoxy comprising at least 80 weight percent of silver flakes.
According to an embodiment of the present invention, said underfill material is a non-conducting adhesive (NCA). Said NCA can be a non-conducting epoxy.
According to an embodiment of the present invention, said drying step is performed in an oven at a temperature below the curing temperatures of both the ICA and the underfill material. According to another embodiment, said drying step is performed at a temperature of maximum 110xc2x0 C.
According to an embodiment of the present invention, said curing step is performed with a curing temperature of maximum 130xc2x0 C.
According to an embodiment of the present invention, said step of aligning and exerting a mechanical pressure is performed in a first tool, and wherein said step of simultaneous pressing and curing is performed in a second tool, after transporting the assembly formed by said substrate and said component from said first tool to said second tool.
According to an embodiment of the present invention, said thermocompression step is performed using a thermode which has a suitable size and shape to exert a uniform mechanical pressure on said component and which comprises a heat source in order to bring said component and said substrate in contact with said component to the curing temperature.
According to an embodiment of the present invention, said curing step is performed by light radiation, preferably UV radiation. Said radiation can be performed in a separate tool.
According to an embodiment of the present invention, the process further comprises the assembling of components onto said substrate by reflow soldering and/or by wave soldering. According to a preferred embodiment of the invention, said reflow soldering and/or said wave soldering are performed after one or several components have been assembled according the process of the invention.
The present invention is equally related to an apparatus manufactured by the process according to any one of the preceding claims.